15. CHECKING THE EQUIPMENT

It is the responsibility of the anaesthetist to check all anaesthetic equipment and drugs before giving an anaesthetic.

There must always be alternative equipment to ventilate the patient’s lungs if the anaesthetic machine or oxygen supply fails. A self-inflating resuscitation bag does not need a source of oxygen. It should be available whenever an anaesthetic is given.

Airway Equipment

An alternative method of ventilating the patient must always be available.

Ideally the anaesthetist would have at least two laryngoscopes of different sizes. The light should be checked. Oropharyngeal (and nasopharyngeal) airways should be available in different sizes. A flexible stylet and gum elastic bougies are excellent aids for intubation. The anaesthetist should have several different sized masks and an appropriate sized endotracheal tube (plus one size smaller and one bigger) available. A laryngeal mask may be used as the airway or as an excellent alternative airway if endotracheal intubation is difficult (secondary plan). Emergency airway equipment (e.g. laryngeal masks, intubating laryngeal masks, percutaneous tracheostomy, fibreoptic laryngoscopes) should be kept together in a labelled container in a central area.

Suctioning

Suction equipment should be available. It consists of a pump to generate a vacuum, a reservoir and tubing. The reservoir must be large enough to hold the aspirated fluid but not too large. (The larger the reservoir the longer it will take to achieve a vacuum). The minimal flow rate should be 35 l/min of air and generate at least 600 mmHg (80 kPa) negative pressure.

Suction may be powered by electricity, compressed gas or by hand/foot.

Continuous Flow Anaesthetic Machine (Boyle’s machine)

The anaesthetic machine can be considered in thee parts: high pressure (pipeline, cylinders, pressure gauges and regulators), low pressure (oxygen failure alarm, antihypoxic device, flowmeters, vaporisers, pressure release valve, and common gas outlet) and the breathing system.

Cylinders and Pipelines

Cylinder and pipeline gases are too highly pressurised (5,000 kPa to 14,000 kPa) for safe flow regulation. Regulators are used to decrease the pressure to a safe level. Pressurised gases must never be connected directly to the breathing system.

(1 atmosphere = 760 mmHG = 98 kPa = 14 psi. 1 psi =6.9 kPa).

Cylinders should be checked regularly for faults. Full and empty cylinders should be kept separately. Cylinders must be handled carefully. They are heavy and oxygen cylinders are a fire risk.

Different gases are supplied at different pressures. Oxygen is stored at 14,000 kPa. A standard D cylinder contains 400 litres, an E cylinder 680 litre and an F cylinder 1400 litres. The gauge pressure on an oxygen cylinder will decrease at a rate proportional to the amount of oxygen used. When half the contents of a cylinder are used, the gauge pressure will be half of the original pressure.

A second oxygen cylinder must always be available and checked.

Oxygen is available as “industrial” or “medical” grade. The same process is used to produce both grades of oxygen and it is safe to use “industrial” grade oxygen if “medical” grade oxygen is unavailable.

Nitrous oxide cylinders are filled with liquid nitrous oxide. The gauge pressure of a nitrous oxide cylinder will not change as the nitrous oxide is used until all the liquid is depleted. Once the gauge pressure of a nitrous oxide cylinder starts to fall the cylinder is nearly empty. A full C cylinder of nitrous oxide contains 450 litres, a D cylinder 900 litres, an E cylinder 1800 litres and an F cylinder 3600 litres.

In order to ensure that the correct cylinder is attached to the yoke of the anaesthetic machine a series of pins on the machine yoke is made to fit an identical pattern of indentations on the cylinder. This is a pin-index system.

Flow Meters

Gases from the cylinders and pipeline pass though flow meters. The flow meters are made for a specific gas. They are not interchangeable. Flow meters have a spindle valve in the base to control flow and a bobbin or a ball in a vertical tube. The bobbin should spin. After the gases pass though the flow meters the different gases are joined together. Oxygen is added last to reduce the chance of giving a hypoxic mixture. New anaesthetic machines link the flow of nitrous oxide to the flow of oxygen to prevent less than 25% oxygen being given (anti-hypoxic device). Anaesthetic machines without an anti-hypoxic device should have an oxygen analyser.

Oxygen Failure Alarm

The anaesthetic machine should have an oxygen failure warning device. An anaesthetist should not use an anaesthetic machine that does not have an oxygen failure warning device or a broken device. If there is no alternative the anaesthetist must check the oxygen gauge pressure every 5 minutes. The cylinder must be changed when the cylinder pressure is less than quarter full.

There are a variety of alarms. Older models depend on batteries to power a red light and nitrous oxide to power a whistle (Bosun oxygen failure alarm). The anaesthetist must check that the batteries are working. Other devices do not rely on batteries and will shut off the nitrous oxide. Some have a reserve supply of oxygen.

Vaporisers

A horizontal pipe (back bar) on the anaesthetic machine connects the flow meters to a common gas outlet. The breathing systems are connected to the common gas outlet. Vaporisers are usually mounted on the back bar. Some older vaporisers may be free-standing and are connected to the common gas outlet. The anaesthetist must check that the vaporisers are connected in the correct direction.

Vaporisers are made for a specific volatile anaesthetic agent. Filling a vaporiser with the incorrect volatile anaesthetic agent will produce the wrong concentration. Some vaporisers have a special filling system to ensure that they are filled with the correct agent. If a vaporiser does become contaminated with the incorrect agent it should be emptied, washed out several times with the correct agent and then blown though with oxygen or air until all smell has been eliminated.

On some anaesthetic machines it is possible to connect more than one vaporiser to the back bar. Newer anaesthetic machines have a mechanism to prevent more than one vaporiser being turned on at the same time. Turning more than one vaporiser on at the same time will produce dangerous concentrations of volatile anaesthetic gases.

The vaporisers made for the back bar are for use with compressed gas. They have a high internal resistance. They must not be used for drawover anaesthesia.

The anaesthetist must check that the vaporiser is filled with the correct agent, correctly fitted to the back bar and that it easily turns on and off. The vaporiser should be left in the off position. (A Boyle’s bottle should have both the lever and the plunger pulled up. Check that filling ports are closed). Vaporisers must never be tilted or turned upside down. This will produce dangerous concentrations of the agent when it is turned on.

Oxygen Flush/Pressure Relief Valve

At the end of the back bar there may be an emergency oxygen flow button (oxygen flush) and a pressure relief valve. Anaesthetic machines should have an emergency high flow rate (20 to 35 litres/min) supply of oxygen that bypasses the flow meters and the vaporisers. The anaesthetist should check the oxygen flush by pressing the spring-loaded button. The pressure relief valve is located downstream from the flow meters and the vaporiser. It protects the anaesthetic machine and vaporisers from high pressures. It does not protect the patient.

Checking the Anaesthetic Machine

Always have an alternative resuscitation device (e.g. self-inflating bag).

Check that cylinders are full and attached to the anaesthetic machine. There must always be a reserve supply of oxygen. Never use a machine if there is no reserve supply of oxygen.

Turn off all cylinders.

Turn on all flow meters. There should be no flow. Check the flow meters for cracks.

Turn on the oxygen cylinder. There should only be flow in the oxygen flow meter. The bobbin should spin. Repeat with each oxygen cylinder. Set the oxygen flow to 4 litres/min.

Turn on the nitrous oxide cylinder. Check that there is flow in the nitrous oxide flow meter (the bobbin should spin) and that the oxygen flow meter is still at 4 litres/min.

Turn off the oxygen supply and push the oxygen flush button.
The oxygen failure alarm should sound.

Turn on the oxygen cylinder again. The oxygen failure alarm should go off.

Check that all vaporisers are full and correctly fitted. The controls should operate thoughout their full range without sticking. Turn off the vaporisers.

If the anaesthetic machine is fitted with a pressure relief valve it should be tested by occluding the common gas outlet whilst gas is flowing. (Never do this test if a
pressure relief valve is not fitted).

Attach the breathing system. Check that it has been correctly assembled. Close the APL valve, occlude the end and fill with gas. Squeeze the reservoir bag to ensure there are no leaks.
Open the valve and ensure the breathing system empties.

Check all airway equipment, suction equipment and drugs.